This invention relates to ion implantation into In-based Group III-V compound semiconductors.
Ion implantation is a well-known technique for doping semiconductors and, in the case of silicon semiconductor devices, is characterized by excellent control of doping concentration and depth as well as precise registration of implanted zones with electrical contacts. It entails causing high energy ions to impinge on and penetrate into a surface of the semiconductor so as to form a shallow region in which the ions are incorporated. To render the ions electrically active, the semiconductor is typically subjected to a suitable heat treatment known as "activation".
In contrast with silicon devices, where ion implantation is a relatively mature technology, ion implantation into Group III-V compounds is still primarily in a developmental stage faced with special problems, many of which are not extant in the silicon art. For example, Group III-V compounds often exhibit nonstoichiometry as a result of evaporation of the more volatile Group V species (e.g., P in InP) during high temperature processing steps (e.g., activation). Nonstoichiometry also results from the disparate masses of the Group III and Group V species (115 for In and 31 for P) which, during ion implantation, cause the P atoms to be displaced deeper into the semiconductor than the In atoms. In either case, Group V vacancies (e.g., P vacancies) occur and present a unique problem because Group II dopant species (e.g., Be) which enter such vacancies are electrically inactive. As a consequence, the implanted region may contain many dopant atoms which do not contribute to the carrier concentration of the semiconductor.
As between GaAs and InP and their related materials system, interest in GaAs for high-speed integrated circuit applications is widespread, but considerable attention is also being given to In-based Group III-V compound electronic devices (e.g., InP FETs) because of their ability to be integrated with optical devices (e.g., InP/InGaAsP lasers, InP/InGaAs photodiodes) made from similar materials, especially those suitable for use in long wavelength fiber optic applications. In addition, InP has higher peak electron drift velocity and higher breakdown field than GaAs, which portends higher performance of electronic devices such as FETs.
Thus, if ion implantation into InP and other In-based Group III-V compounds can be mastered, it promises to have significant device implications. The following publications attest, however, to the hurdles workers have faced and failed to surmount in their attempts to realize a practical ion-implantation process in InP:
1. J. P. Donnelly et al, Appl. Phys. Lett., Vol. 31, p. 418 (1977). PA0 2. T. Inada et al, J. Appl. Phys., Vol. 52, p. 6623 (1981). PA0 3. K. V. Vaidyanathan et al, Nucl. Instru. and Methods, Vol. 182/183, p. 631 (1981). PA0 4. J. D. Oberstar et al, J. Electrochem. Soc., Vol. 129, p. 1312 (1982). PA0 5. A. N. M. Masum Choudhury et al, Appl. Phys. Lett., Vol. 43, p. 381 (1983). PA0 6. B. Molnar et al, Mat. Res. Soc. Symp. Proc., Vol. 27, p. 329 (1984). PA0 7. J. P. Donnelly, Nucl. Instru. and Methods, Vol. 182/183, p. 553 (1981). PA0 8. U. Konig et al, J. Elec. Mat., Vol. 14, p. 311 (1985).
These studies have shown that a peak carrier concentration of about 10.sup.19 /cm.sup.3 is achievable with n-type implants, but for p-type implants the electrical activation is usually less than 50% in the 10.sup.17 /cm.sup.3 range, but is only about 15% at the maximum achievable hole concentration of about 10.sup.18 /cm.sup.3. In addition, broadening of the implant profile (Inada et al) as well as in-diffusion of p-type dopant was observed after an elevated temperature activation process (Oberstar et al and Konig et al). The lower electrical carrier concentration as well as the in-diffusion makes control of the p-type implant difficult and realization of a shallow p.sup.+ -layer a challenge. Such a p.sup.+ -layer could be used to form an abrupt p-n junction of a field effect transistor, a main junction of an avalanche photodiode, or a low contact resistance layer of a laser diode.
Looking deeper at the problems of forming a shallow ion-implanted p.sup.+ -layer in InP, the first four references above relate to ion implantation of Be, Mg, Zn or Cd ions, typical p-type dopants, into Fe-doped, semi-insulating InP substrates, and report the following: (1) p.sup.+ -implants using Mg, Zn or Cd resulted in a dead surface layer without free electrical carriers after activation; (2) elevated temperature activation of Be and Zn implants caused in-diffusion of the dopants; (3) the peak carrier concentration saturated at about 10.sup.18 /cm.sup.3 after activation, as noted above; and (4) the implant profile was broadened. The dead surface layer is correlated with the heavily damaged surface associated with heavy ion implants (e.g., Mg, Zn). To eliminate the dead surface layer, an elevated temperature (hot) implant was proposed for Mg and Zn. However, a more graded implant tail was observed as the result of the hot implant, especially for a shallow p.sup.+ -implant (Inada et al).
Regarding the in-diffusion of Be, Molnar et al conclude that most Be atoms in the diffusion front were in electrically inactive sites. They suggest, therefore, that in-diffusion is not a problem and furthermore that it can be eliminated by suitable RTA. However, our attempts to reproduce their experiments indicate the contrary: the in-diffusion was not eliminated by RTA and Be was located on electrically active sites. The mechanism for the Be in-diffusion is not clear, but theory suggests it may be related to stoichiometric disturbances or bulk impurity redistribution during implantation and/or annealing.
In contrast, in the GaAs art workers have tried to control in-diffusion of the dopant species and to enhance peak carrier concentration and activation efficiency by pre-implanting with As ions followed by implantation of acceptor ions such as Zn. See, for example, an article by D. E. Davies et al in App. Phys. Lett., Vol. 44, No. 3, pp. 304-6 (1984). Despite these results, the only known attempt to apply co-implantation to In-based Group III-V compounds (As and Zn co-implantation into InGaAs) showed no increased peak carrier concentration or profile abruptness over the use of Zn implantation alone. See, E. V. K. Rao et al, Jap. J. Appl. Phys., Vol. 25, No. 6, pp. L458-461 (Jun., 1986), especially FIG. 2(a) where the Zn-alone and Zn/As co-implant profiles are shown to be nearly identical.